Bad/awkward mistake and dont know what to do or say!!..any advice??....:/?

i am really gud friends with this lad but he lives over in england...he comes over to his family a couple of times a year nd we hangout ...during x-mas just gone we were up in his house but decided to go down to his uncles for a few drinks , we were having a good nyt or what ever and his aunty and uncle went to bed, leaving just us alone downstairs. we were talking for a while and one thing led to another and it ended in us sleeping together.....i have never thought anything like that would have happened between us as i only see him as a gud friend!!...since then we havnt spoken nd never speak on facebook ....im just wondering what i could do to try rebuild the friendship that we had , because at the moment i tink the both of us feel really awkward about it and are avoiding bringing it up......i also feel really guilty as he has a girlfriend and they were going out the time this all happened!!...any suggesstions will be greatly appriciated..thanks in advance..